In the press section of a web-formation machine, such as a paper or board machine, a great deal of water is removed from the web. The water is removed using a press nip formed from two rolls. Usually the web is led to the press nip between two fabrics. After the press nip, the web is guided onwards in the web-forming machine, supported by a fabric.
Before the press section or the press nip, the edges of the web are evened, usually by water cutting. In addition, the edge parts of the fabric become dirty more rapidly than the rest of the fabric. The previous processing of the web also make the edges different to the rest of the web while the edge parts also stretch. Thus, the edges of the web tend to adhere to the first fabric, even though the rest of the web is guided to another fabric after the press nip using the vacuum effect of a suction roll. Usually the edges of the web detach from the fabric after a delay, which further stretches the edges and causes flutter in the web. The flutter is also increased by the low tension in the web and the airflow caused by the fabrics. In addition, centrifugal force in the area of the suction roll further increases the stretching of the edges. The stretched and even creased edges of the web must be cut off later, which reduces production efficiency. In the worst case, the irregular edges of the web may even cause the web to break during transfer, or during processing in the paper or board machine. The irregular and stretched edges are a major problem particularly when coating the web.
Attempts have been made to eliminate the problem described by increasing the vacuum in the suction roll and altering the tension of the fabrics. Different kinds of fabric have also been tried. Despite these attempts, the problem still persists, or at least it appears soon after changing a new fabric. In practice, increasing the level of vacuum in the suction roll has little effect, as the vacuum effect only extends to the very surface of the fabric. Thus it is nearly impossible to use a suction roll to pull an edge that has already separated back onto the surface of the fabric. The problem is exacerbated when the velocity of the web is increased.
European patent application number 1101864 discloses an arrangement in the press section of a paper machine. The core invention relates to a felt guide roll transfer in transferring the web from the press felt to the next press felt after a suction roll. In the second embodiment of the invention, a suction box is disclosed between the press nip and the suction roll, which is intended to prevent the re-wetting of the press felts, and thus the web, after the press nip.
The suction box presented is long. In that case, the production of a vacuum in the full-width suction box consumes much energy. Due to friction, the large suction box also acts as a brake on the press felt and wears it rapidly. The suction box has a low controlling effect on the web, as there is an obvious discontinuity point in the vacuum effect in the machine direction, due to the mutual positioning of the suction box and the suction roll. It is also difficult to set a large suction box in the correct position while its support requires large structures in an otherwise cramped position.